1. Field of the Invention
The invention relates to a micro-gripper for accommodating and holding micrometer-scale or sub-micrometer-scale objects and to a method for the production thereof.
2. Description of the Prior Art
Micro-grippers are currently used in numerous fields in the context of microsystem technology. Thus, micro-grippers are used, for example, in microtechnology and nanotechnology for manipulation and mounting or for joining extremely small objects. Further areas of application for micro-grippers are found in physics, biology, chemistry, and medicine, micro-grippers being used, for example, in the context of the analysis and assaying of samples for accommodating, gripping, and holding the samples.
The micro-grippers typically comprise a base body, which is connected to two or more movable and/or elastic gripping elements, which are used to accommodate and hold an object. At least one actuator provided on the micro-gripper is typically provided for the active actuation of the gripping elements, which moves the gripping elements, which act toward one another. In micro-grippers of this type, the gripping forces acting on the object may be partially set or regulated by a corresponding activation of the actuators. Micro-grippers without actuators are also known, in which the accommodation and holding of an object does not occur actively, but rather passively with exploitation of elastic material properties. Micro-grippers of this type have at least two opposing gripping elements spaced apart from one another, between which an object may be clamped. The gripping elements are elastically deformed by the clamping of an object between them, and thus generate elastic retention forces reacting onto the object.
It is to be noted here that for the following statements, the term micro-gripper is restricted to the versions having only two gripping elements.
Thus, an embodiment of a micro-gripper for micro-mounting is disclosed in the publication DE 195 23 229 A1, which comprises a base body, a piezotranslator fastened to the base body as a linear actuator, and a microstructure body connected to the base body and the piezotranslator. Two opposing gripping elements acting toward one another and a mechanical lever transmission having bending joints for the enlarging transmission of a linear movement of the piezotranslator onto the gripping elements are constructively unified in one component in the microstructure body. Upon a length change of the piezotranslator, the elastic bending elements deform and thus initiate a targeted movement of the gripping elements away from or toward one another.
The production of the microstructure body is performed from a (100) silicon wafer polished on both sides having a thickness of 240 μm using the known structuring processes, lithography, and anisotropic etching. The microstructure body is subsequently fastened on the base body, a silicon substrate, using adhesive in such a way that only selected small contact areas between the microstructure body and the base body are glued, the bending elements and the gripping elements being axially displaceable along the contact faces. The production of the microstructure body from silicon, especially its good ability to be micro-structured and the lack of plastic deformation of silicon, is especially emphasized as the essential advantage of the described micro-gripper. In addition, the possibility exists of attaching piezoelectric, for example, piezoresistive layers to the gripping elements, in particular to their particular gripping faces, to convert the gripping force into an electrical signal and thus to adapt the gripping force to the object to be gripped. Further embodiments of micro-grippers which use piezosystems as actuators may be inferred, for example, from the publications DE 196 48 165 A1 and DE 101 14 551 C1.
In addition to piezo systems, other elements or principles are also used to move the gripping elements. Thus, a micro-gripper may be inferred from the publication DE 197 15 083 A1, in which flat coils or permanent magnets of an electromagnetic drive are integrated in a yielding gripping mechanism. The closing of the gripping elements is caused by applying an external magnetic field. The publications US 2004/0004364 A1, US 2002/0061662 A1, and U.S. Pat. No. 5,149,673 A, in contrast, describe micro-grippers whose gripping elements may be moved using electrostatic attractive or repulsive forces. Finally, a nanogripper is disclosed in the publication US 2005/0029827 A1, in which the gripping elements may be moved by exploiting electro-thermal material properties. The gripping elements are connected via a jointed mechanism to elements which heat as a result of a current flux, expand, and cause a movement of the gripping elements via the jointed mechanism.
The grippers cited up to this point all have the disadvantage that it is necessary to feed electrical or magnetic energy to the actuator to grip and hold objects. If this is interrupted or disturbed, a failure of the gripper occurs, that is, the object may detach from the micro-gripper in an undesired way. In addition, the necessary electrical adaptation of the micro-gripper in the required dimensions in the micrometer range is complex and costly.
In particular, any contamination of the sample by the micro-gripper, for example, by a material transfer from a preceding sample to the next sample, is to be prevented for applications of micro-grippers in the scope of material analysis of micrometer-scale or sub-micrometer-scale samples. Therefore, for example, to grip and hold samples in the context of study, in particular using electron microscopes, such as transmission electron microscopes (TEM) or scanning electron microscopes (SEM), new micro-grippers are used for each sample. Because micro-grippers are only intended for a single use for purposes of this type, they are to be producible cost-effectively as mass produced products. Nonetheless, they must fulfill all the requirements to ensure reliable gripping and holding in such applications. The micro-grippers described above are not or are not optimally suitable for this purpose for the cited reasons, however.
In addition to the micro-grippers having actuators described above, a micro-gripper is described in the publication US 2002/0166976 A1, which is particularly also suitable for use for gripping and holding a sample in the context of studies using TEM. The micro-gripper described therein comprises a rod-shaped or cylindrical elongate body, which has one or more receptacle slots open on three sides on one end in such a way that a sample may be clamped in the receptacle slot. The accommodation and holding of the sample is based, as described above, on the elastic deformation of the material surrounding the receptacle slot and the restoring force thus caused, which acts on the clamped sample.
The following method may be inferred from the cited document for producing the micro-ripper. A piece of linear tungsten wire having a wire diameter of 50 μm is used as the starting material. In a first method step, an end area of the tungsten wire is initially processed using electropolishing or an etching method in such a way that the tungsten wire tapers in this end area toward the wire end to a diameter of a few micrometers. In a second work step, a receptacle slot open on three sides toward the end of the tapered area is worked out of the now conically tapering end area of the tungsten wire using an ion beam incident perpendicularly to the longitudinal axis of the tungsten wire. In a third method step, in addition to the first receptacle slot, for example, a second receptacle slot rotated by 90° thereto may be worked out by a further application of the ion beam. At least two or four gripping elements, between which an object may be clamped, thus arise in the tapered end area of the tungsten wire. The receptacle slot has a width of 2 μm and a depth of 30 μm according to one exemplary embodiment.
The micro-gripper disclosed in US 2002/0166976A1 has the disadvantage that it is not technically possible by the specified production method to produce the faces adjoining the receptacle slot, that is, the internal clamping faces on the gripping elements, exactly parallel to one another. Rather, by applying the ion beam to produce a receptacle slot, the receptacle slot is wider on the side facing toward the ion beam than on the side facing away from the ion beam. The clamping faces defining the slot which are thus generated are therefore not oriented parallel to one another. This finally results in an uneven distribution of clamping forces on the object to be held and has the danger that objects clamped between the clamping faces may shift in relation to the micro-gripper. Furthermore, the described method is only suitable in a limited way, and/or not at all for mass production, because exact fixing and positioning of the tungsten wire is required individually for each micro-gripper, before the processing using the ion beam may be performed, which makes the production method time-consuming and costly.
A production method of partially movable microstructures based on a dry-chemical etched sacrificial layer may be inferred from DE 195 22 004, the sacrificial layer, which typically comprises polyimide, being applied directly to a substrate layer, on which, completely spaced apart from the substrate layer by the sacrificial layer, a later partially movable micro-structured material layer is applied, for example, in further implementation as a cantilever with or without additional tip. For the purposes of the partial movement capability, the sacrificial layer located between the cantilever layer and the substrate layer is only partially removed in the course of a dry-chemical etching method, so that a residue of sacrificial layer remains in existence as a type of spacer.